A unique PSK transceiver, Version 2

By Steven Weber, KD1JV

3/1/11

NOTE: PA3CNO discovered and devised fixes for serveral problems with the design as discribed below. I will eventually include his modifications to this page, but for now please see his blog for details on the problems he found and his fixes.

The original "unique" PSK transceiver proved that an efficient Class C power amplifier can be used to transmit PSK signals, although it requires a bit of front end circuitry to do so. The concept was simple. Reconstruct the AM modulation envelope from the PSK signal created by the PC sound card to do the carrier modulation of the Class C amplifier, then detect the zero crossings to change the phase of the signal driving the PA. I've since learned that changing the phase of the signal at every amplitude zero crossing is not the way PSK works. The phase only changes between zeros, or going from a zero to a one. The phase does not change when going from a zero to a one. However, it seems most PSK decoders will still decode the PSK data word if the phase changes at every amplitude crossing, though it may not be 100% reliable. This is because the software will look at the bit pattern of the data word and make a "best fit" guess at what the word is, and most of the time gets it right.

The main disadvantage of the original design is that it operates at a fixed audio frequency, therefore requiring it to be tuned exactly to another stations frequency, rather than being able to simply click on the PSK waterfall and automatically go there as is the case when using a SSB rig. It also required setting the T/R offset rather precisely to exactly 1 KHz, the frequency the amplitude envelope detector worked at. This Version 2 of the unique PSK transceiver addess those issues with the following improvements:

The receiver is now a super-heterodyne with USB crystal filter. Not only does this eliminate potential opposite sideband QRM, the overall receiver sensitivity is improved over the basic Direct Conversion receiver used in the original design. I can clearly "see" a 0.2 uV signal in the PSK waterfall.

The AM modulation envelope recovery detector is simplified and now works over a reasonably wide range of frequencies.

The transmit frequency is now set by using a "SPOT" switch which enables the transmitter mixer and driver stages so the frequency can be seen on the PSK waterfall. A ten turn pot is used to adjust the frequency of the transmitter to match the signal you see on the waterfall of the station you want to contact, or to put set the frequency to a clear spot on the band. Although this takes a few seconds to do, you know exactly where your transmitted signal is.

The AM envelope modulator is changed to use a P-Channel MOSFET pass transistor instead of a PNP.

The new circuitry uses less parts then the original design, so can be built on a smaller circuit board.

Circuit description:

The receiver is a basic SA612 circuit. (U5 and U6) The LO for 20 meters is about 9 MHz and an IF of 5.068 MHz. The IF filter uses two crystals and has a peak response at about 5.070 MHz, so this matches up well with the 20 M PSK sub-band centered at about 14.071 MHz. The BFO frequency is adjusted to be below the IF frequency so Upper Side Band reception results. This was done because it was easier to pull the transmitter VXO higher than the receiver LO frequency to get the proper beat notes in the audio band than it was to go the other way around. The detected audio output of the BFO mixer is amplified by a simple LM386 amplifier (U2). The additional gain cap between pins 1 and 8 was found not to be needed for good sensitivity.

The transmitter uses another SA612 (U6) for mixing the BFO oscillator with a 9 MHz VXO signal produced by the SA612. The VXO uses two crystals in parallel for increased range, along with a series inductor. The frequency is voltage tuned using a 10 turn pot and 1N4756 Zener didoe used as a tuning diode, which gives enough capacitance swing to tune the VXO about 1.5 kHz, using HC-49US crystals. This tuning range matches the band width of the receiver's crystal filter pretty closely.

The transmitter mixer output is filtered by a double tuned circuit, using modified 10.7 MHz IF cans. The modification is simply breaking out the cap in the bottom of the can, so it can be retuned to 14 MHz using an external cap. A two stage transistor amp (Q4, Q6) brings the signal level up enough to drive the gate of a 74HC86 XOR gate (U4). The XOR gate is used to change the phase of the transmitted signal and to drive the gates of the three BS170 MOSFETs used for the power amplifier. Two gates are connected in parallel to improve drive to the PA gates.

During transmit, the PSK audio signal from the PC first goes into a peak detector (U4a). A resistor across the peak detector holding cap induces some ripple so that the detector can follow the modulation signal on the falling side of the signal. This ripple is removed by a following low pass filter (U4b). The resulting AM modulation signal is then both voltage and current amplified and used to power the RF PA (U4d/Q10).

A voltage comparator (U4c) is used to detect the zero crossings of the AM modulation signal from the low pass filter stage. The output of the comparator is used to clock a "D" flip-flop (U5a) which in turn is used to change the phase of the transmitted signal via the 74HC86 driving the PA.

Transmit condition is detected by picking a signal off the AM modulation output transistor, which has a sufficient level to turn on a 2N7000 MOSFET (Q5). The output of Q5 mutes the audio output to the PC, disconnects the receiver input from the transmitter LPF and applies power to the transmit circuits via a PNP transistor switch (Q2).

A 4049 hex inverter (U6) is used to produce a negative supply voltage for the op amps, so that they don't clip on the negative swing of the input signal. Adding an additional 2 diodes and caps created a voltage doubler to increase the voltage available to supply the VXO tuning pot and increase the range of the VXO. Although silicon 1N4148 are shown used on the schematic, using shotky diodes such as 1S101's or 1N5811's will produce a somewhat higher votage output.

A 78L05 three terminal regulator supplies regulated supply voltage to most of the circuits.

(Note: to pint schematic, first right click on image and save to your HD. Then print from a graphics viewing program)

Construction:

It is best to built this rig on a printed circuit board, if you know how to make one. The PCB layout is available HERE as a pdf file which can be printed to actual size. This is a "through board" view, suitable for direct toner transfer, as the transfer will invert the image when it is ironed onto the board. Flood fill is used to maximize the ground plane area, (and reduces the amount of enchant used). One must be careful to fully etch the board so there are no shorts between the pads and tracks to the ground plane and to be careful when soldering not to make solder shorts to the ground plane. since this is a single sided layout, there are a few point to point wire jumpers required. Some of these are straight line and so can be made with bus wire. The longer jumpers will need to be made with insulated wire. NOTE: Q10 should have a heat sink on it.

Board component overlay:

If you do build this rig, you should have at least a 20 MHz oscilloscope. This will be needed for adjusting the transmitter band pass filter and verifying all the circuits are working properly. A frequency counter will also be handy and of course, your trusty DVM.

Tune up:

Before applying power for the first time, check for shorts on the 12 supply bus and the 6 volt bus. Now apply power and verify presence of the regulated 6 volts and that the negative voltage supply is present on pin 11 of the LM324. Using a freq counter or general coverage receiver, set the BFO trimmer so the BFO frequency is about 5.068 MHz. Measure this on pin 7 of the BFO SA612 (U8) . Activate the transmitter mixer and buffer amps by turning on the Spot switch. With a scope on the audio output, you should see a signal. You may have to tweak the 1st mixer LO trimmer (C20). Adjust the cores in the transmitter band pass filter coils (T2/T3) for maximum signal, then peak the receiver input coil (T1).

Turn off the spot switch and connect up an antenna. Connect the audio output to a PC running PSK software. Adjust the BFO trimmer (C27) so that the maximum background noise is centered around 2 kHz on the waterfall. If the band is very quiet, you might have to generate some wide band noise by turning on a vacuum cleaner or turning up the waterfall gain. It is likely you will also see PSK signals on the waterfall. Activate the spot switch again and set to the minimum frequency. Adjust the LO trimmer (C20) so that the beat note is about 1 kHz. Now you can check your transmit frequency tuning span by turning the pot to the other end.

The rig should now be operating more or less in the middle of the most active part of the PSK sub-band window.

Setting the transmit audio level.

This is the tricky part. Connect up a watt meter and dummy load. Activate "tune" mode in the PSK software, which should generate a 1500 kHz tone of constant amplitude. Increase the transmit audio level and at some point the transmit LED (D11) should come on and you should see power output. Increase the audio level until the power output no longer goes up and should be about 3 watts with a 13.8V supply.

For the final audio level adjustment, you will need a 'Scope as I could not think of a simple way to detect when the modulation signal started to "flat top",as I had used up all the available room on the circuit board and did not want to make it any bigger!

Connect the scope to the output of Q10. Click the transmit icon on the PSK window to generate a PSK idle tone at about 2000 Hz. Adjust the transmit audio level so the modulation signal just begins to flat top. Then use the scope to check the waveform at the antenna terminal. If it is doing some crazy things and not matching the modulation waveform, you might have to tweak the Tx band pass filter a little.

Check the levels at the extremes of your operating window, 1 kHz to 3 kHz. Hopefully, the levels will stay the same. With the PC I'm using for bench testing, the audio levels are not consistent over the frequency range. The transmitted signal seems to decode fine even when there is a large amount of flat topping in the modulation and this results in higher average power output, so maybe its not a bad thing. With a SSB transmitter, over modulation would result in spatter, but this is not a problem with the Class C amp used here.